Conventionally, as a developer material employed in electrophotographic systems, a two component developer material, consisting of a carrier and a toner, has been widely employed.
In recent years, as a result of improvements in carrier durability, image quality, and especially fine line reproducibility, a resin coated carrier (hereinunder referred to as coated carrier) is primarily employed. However, during repeated image formation, a coated resin is worn and peels off, and the magnetic particles employed as a core are exposed on the surface.
As a result, the charge applying capability of a carrier to a toner is markedly deteriorated, and Background Staining and toner in-machine scattering are caused. Furthermore, a so-called toner spent phenomenon is caused in which a part of the toner component adheres to the surface of the carrier. Specifically during continuous copying, because the toner and the carrier are subjected to large shearing force, the coated resin is readily worn and peels off, and in addition, the toner spent phenomenon is accelerated. Because the durability of a two component developer material, in most cases, depends mainly on the carrier, improvement in carrier durability becomes a major goal.
Silicone resin has received attention, as a highly durable coating resin. Because silicone resin exhibits a three dimensional structure, abrasion resistance is markedly improved. Furthermore because the silicone resin is a low surface energy resin, the toner spent phenomenon is hardly caused to result in high durability.
However, because the silicone resin-coated carrier generally exhibits low charge application capability, Japanese Patent Publication Open to Public Inspection No. 6-118724 proposes a coating resin hardened by incorporating a methylated melamine resin into the silicone resin, and Japanese Patent Publication Open to Public Inspection No. 7-28281 proposes a coating resin in which a siloxane compound having a specified structure is added into the silicone resin. However, because it is difficult to disperse these compounds uniformly into the silicone resin, a carrier resulting in uniform chargeability is not obtained.
Furthermore, Japanese Patent Publication Open to Public Inspection No. 64-91144 proposes a method in which a carrier is subjected to mechanical agitation to enhance the charge amount. However, in this method, until the desired charge amount is obtained, mechanical impact is continuously applied. Thus, as a result of the application of the excessive impact, the silicone coating resin of the carrier is damaged. When copying is repeatedly carried out employing the resulting carrier, the charge amount of the developer material decreases to result in the formation of background staining and an increase in toner scattering. Namely, the durability of a developer material is unexpectedly degraded and therefore, this method is not a practical means.